In recent years, for example, regulations on the cleaning of exhaust gas of a diesel engine have become severer and the combustion phenomenon of a diesel engine has been made clearer. With this, to reduce diesel particulates typified by black smoke for the purpose of cleaning exhaust gas exhausted from an engine, it is important to transform fuel injected from the injection portion of a fuel injection nozzle into fine particles of absolute minimum. To further enhance the transforming of fuel into fine particles, it is effective to intensify the injection pressure of fuel.
However, pressure intensified in the fuel injection system for a diesel engine mounted in a vehicle such as an automobile is approaching a limit. For example, also in a common rail type fuel injection system, a request to intensify injection pressure of fuel has become very severe and a request for a value exceeding a limit of resistance to pressure of a supply pump for pressure-supplying fuel to a common rail has been made. U.S. Pat. Nos. 5,682,858 and 6,752,325 show a pressure intensifying piston type fuel injection apparatus for intensifying the injection pressure of fuel to be injected into the cylinder of the engine from an injector to a value larger than the pressure of fuel accumulated in a common rail.
The apparatus, as shown in FIG. 10, is provided with a common rail 101 for accumulating fuel pressure supplied by a fuel injection pump (not shown), a pressure intensifier 102 for intensifying fuel supplied from the common rail 101, a fuel injection nozzle 103 for injecting high-pressure fuel having pressure intensified to a value higher than common rail pressure by the pressure intensifier 102, and a solenoid valve 105 for performing the control of intensifying the pressure of the pressure intensifier 102 and control of opening or closing the fuel injection nozzle 103. Then, the pressure intensifier 102 has a pressure intensifying chamber 111, which is partitioned in a hydraulically hermetic manner by a pressure intensifying piston 110 and a cylinder, a piston back pressure chamber 112, and a piston control chamber 113. Here, the pressure intensifying piston 110 is so constructed as to lift to a side to increase the hydraulic pressure of fuel in the pressure intensifying chamber 111 when the hydraulic pressure of fuel in the piston back pressure chamber 112 becomes higher than the hydraulic pressure of fuel in the piston control chamber 113.
Then, when the hydraulic pressure of fuel introduced from the pressure intensifying chamber 102 into a fuel reservoir exceeds a nozzle opening pressure, the fuel injection nozzle 103 is so constructed as to lift to a side to cause a nozzle needle to open a valve. Here, the nozzle opening pressure is set on the basis of force obtained by adding the biasing force of a spring to the hydraulic pressure of fuel in the nozzle back pressure chamber. Then, the pressure intensifying piston type fuel injection apparatus is integrally provided with the pressure intensifier 102, the fuel injection nozzle 103, and the solenoid valve 105 to construct an injector and a hydraulically operated 2-position 3-way switching valve 104 is built in this injector.
The spool valve 114 of this 2-position 3-way switching valve 104 has a first position capable of introducing fuel discharged from the common rail 101 into the piston control chamber 113 of the pressure intensifier 102 and the nozzle back pressure chamber of the fuel injection nozzle 103, and a second position capable of returning fuel flowing out of the piston control chamber 113 of the pressure intensifier 102 and the nozzle back pressure chamber of the fuel injection nozzle 103 to the low pressure side of a fuel system (fuel tank 107). Then, when the hydraulic pressure of fuel in the pressure control chamber 115 is large, the spool valve 114 of the 2-position 3-way switching valve 104 is set at the first position by the biasing force of a spring 116 and when the hydraulic pressure of fuel in the pressure control chamber 115 is small, the spool valve 114 of the 2-position 3-way switching valve 104 is set at the second position against the biasing force of the spring 116.
The solenoid valve 105 has a solenoid valve chamber 117 built therein and is so constructed to perform the control of increasing or decreasing the hydraulic pressure of fuel in the pressure control chamber 115 to switch the position of the spool valve 114 of the 2-posiiton 3-way switching valve 104. Here, a valve 120 operating integrally with an armature 119 is housed in the solenoid valve chamber 117. Then, the solenoid valve 105 has a solenoid coil 121 for driving the valve 120 in the direction to open the valve and a spring 122 for biasing the valve 120 in the direction to close the valve.
Then, in the injector are formed a first fuel introduction path 131 for introducing fuel from the common rail 101 via the switching valve chamber 123 of the 2-posiiton 3-way switching valve 104 into the piston control chamber 113 of the pressure intensifier 102 and the nozzle back pressure chamber of the fuel injection nozzle 103 and a second fuel introduction path 132 for introducing fuel from the common rail 101 via the piston back pressure chamber 112 of the pressure intensifier 102 and the pressure intensifying chamber 111 of the pressure intensifier 102 into the fuel reserving part of the fuel injection nozzle 103. Then, a first fuel introduction path 133 branched from the first fuel introduction path 131 at a portion closer to the upstream side in the direction of flow of fuel than the switching valve chamber 123 of the 2-posiiton 3-way switching valve 104 introduces fuel from the common rail 101 into the pressure control chamber 115 of the 2-posiiton 3-way switching valve 104.
Then, in the injector are formed a first fuel discharge path 141 for returning fuel from the piston control chamber 113 of the pressure intensifier 102 and the nozzle back pressure chamber of the fuel injection nozzle 103 via the switching valve chamber 123 of the 2-position 3-way switching valve 104 to the fuel tank 107 and a second fuel discharge path 142 for returning fuel from the pressure control chamber 115 of the 2-posiiton 3-way switching valve 104 via the solenoid valve chamber 117 of the solenoid valve 105 to the fuel tank 107. Then, a downstream end in the direction of flow of fuel of the second fuel discharge path 142 is connected to the first fuel discharge path 141 at a portion closer to the downstream side in the direction of flow of fuel than the switching valve chamber 123 of the 2-posiiton 3-way switching valve 104. Then, the first fuel discharge path 141 at a position closer to the downstream side in the direction of flow of fuel than a merging portion 143 where return fuel flowing through the first fuel discharge path 141 merges with return fuel flowing through the second discharge path 142 is connected to a return pipe 106 via the leak port of the injector. The return pipe 106 is a fuel return pipe line for merging the flow of return fuel flowing out of the piston control chamber 113 of the pressure intensifier 102 and the nozzle back pressure chamber of the fuel injection nozzle 103 with the flow of return fuel flowing out of the solenoid valve chamber 117 of the solenoid valve 105 to return the flow of return fuel collectively to the fuel tank 107.
However, in the pressure intensifying piston type fuel injection apparatus, from the operating principle of the pressure intensifier 102, the amount of flow of return fuel more than [(pressure intensifying ratio−1)×the amount of fuel injection (to which the amount of static leak flowing out of the respective sliding portions of the injector and the amount of switching leak caused by the 2-position 3-way switching valve 104 and the solenoid valve 105 (the amount of dynamic leak) are applied] is produced. The return fuel flows out of the leak port of the injector during the period of fuel injection and is returned via the return pipe 106 to the fuel tank 107. For this reason, as shown in FIG. 11, a large positive pressure is developed in the first and second fuel discharge paths 141, 142 and the return pipe 106 (hereinafter referred to as the pressure fluctuation of return fuel).
That is, in the pressure intensifying piston type fuel injection apparatus, there is a possibility that with an increase in the amount of flow of return fuel and an increase in the pressure of return fuel, the pressure fluctuation of return fuel (low-pressure side pressure fluctuation) which does not become a problem in an injector used for a usual common rail type fuel injection system propagates to the solenoid valve chamber 117 of the solenoid valve 105 and exceeds the limit of resistance to pressure of the sealing part such as an O-ring for preventing fuel from leaking from the solenoid valve chamber 117 to the outside. Then, when the apparatus is fastened by screwing to the fuel injection nozzle 103 via the sealing part of the solenoid valve 105, there is a possibility that the sealing part of the solenoid valve 105 is broken (for example, the O-ring is broken) to cause fuel to leak from a portion fastened by screwing. With this, the solenoid valve 105 needs to be further enhanced in resistance to pressure. Hence, there is presented a problem of increasing the cost of the system.